Among measures to alleviate a future shortage of frequency resources is the secondary usage of frequencies, which has been being vigorously studied. The secondary usage of frequencies means that all or a portion of frequency channels allocated to one system with a higher priority are secondarily used by another system. In general, a system to which frequency channels are allocated with a higher priority is called a primary system, while another system that secondarily uses the frequency channels is called a secondary system.
TV white spaces are an example of frequency channels that are expected to be secondarily used. TV white spaces refer to frequency channels that are allocated to a TV broadcasting system serving as a primary system, but are not locally used by the TV broadcasting system. By permitting a secondary system to use TV white spaces, efficient utilization of frequency resources may be achieved. There are a plurality of standards for wireless access schemes in the physical layer (PHY) and the MAC layer that allow for the secondary usage of TV white spaces, such as IEEE 802.22, IEEE 802.11af, European Computer Manufacturer Association (ECMA)-392 (CogNea), and the like.
The IEEE 802.19 working group has focused on smooth coexistence of a plurality of secondary systems that employ different wireless access schemes. For example, in the IEEE 802.19 working group, functions for coexistence of secondary systems are divided into three functional entities, i.e., coexistence managers (CM), coexistence enablers (CE), and coexistence discovery and information servers (CDIS). CMs are a functional entity that mainly makes a decision for coexistence. CEs are a functional entity that serves as an interface for mediating transmission of commands or exchange of information between a CM and a secondary usage node. CDISs are a functional entity that serves as a server for managing information of a plurality of secondary systems in a centralized manner. CDISs also have the neighbor discovery function of discovering neighboring secondary systems that may interfere with each other.
For such functional entities, PTL 1 described below discloses a technique of avoiding concentration of loads on a CDIS that occurs due to neighbor discovery which is performed by a plurality of functional entities in cooperation with each other.